Add ESLint configuration

Author: dtolnay

book/eslint.config.mjs

@@ -0,0 +1,8 @@
+import pluginJs from '@eslint/js';
+
+/** @type {import('eslint').Linter.Config[]} */
+export default [
+  { ignores: ['build/*'] },
+  { files: ['**/*.js'], languageOptions: { sourceType: 'commonjs' } },
+  pluginJs.configs.recommended,
+];